1. Field of the Invention
The present invention relates to a sheet processing apparatus for processing sheets and an image forming apparatus having the sheet processing apparatus.
2. Description of the Related Art
A sheet processing apparatus is usually arranged on a downstream side of an image forming apparatus body, such as a copying machine and a printer, for performing predetermined processing, such as alignment and stapling, on a sheet outputted from the image forming apparatus body to have images formed thereon.
Recently, such sheet processing apparatus has been multifunctionalized, and for example, a sheet processing apparatus may include a side stitching unit by which one side of a sheet bundle made up of the same size sheets is stapled for book binding. In addition, there has been known an apparatus having a saddle stitching unit by which after the center of a sheet bundle made up of the same size sheets is stapled, the sheet bundle is folded back along the stapled position for book binding. Furthermore, as disclosed in Japanese Patent Laid-Open No. 2001-260564, an apparatus is known having a function flattening the curvature of the spine of a sheet bundle saddle stitched and folded. It is also known to bind one side of a sheet bundle with an adhesive such as glue.
On the other hand, color copying machines and color printers have been increasingly used as an image forming apparatus for outputting sheets. It is desirable to provide a more attractive appearance in the sheet bundle resulting from a sheet processing apparatus used in such an image forming apparatus.
However, in the sheet bundle bound with staples as mentioned above, staple wire is exposed on the surface of the sheet bundle, giving an impression in appearance of a simple and unattractive book binding process. Specifically, in the side stitching, on a spine made up of cut edges of sheets, characters or images cannot be recorded. In the saddle stitching, even when a spine is formed by the flattening processing, for example, characters or images recorded on the flattened spine may be hidden behind staple wire.
In order to avoid these disadvantages, a perfect binding method may be employed, in which a sheet bundle is secured with an adhesive instead of staple wire; however, the quality of the appearance may be reduced due to the adhesive. There is also a method, in which a secured sheet bundle is covered with a book cover sheet. However, these methods, such as the perfect binding and the book sheet covering, are generally employed in off-line processing, so that the cost of the product is increased, the apparatus is increased in size, and the operation is complicated. Japanese Patent Laid-Open No. H01-301291 and No. 2004-305879 describe a method for covering a sheet bundle with a book cover sheet, in which the accuracy in folding the sheet bundle along a fore edge of the sheet bundle upon covering the book cover sheet has an influence on the degree of the product appearance. The fore edge of the sheet bundle herein means the open-side edge of the sheet bundle (opposite to the spine). The mechanism for folding the book cover sheet is not described in these Patent Documents.